1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator which includes a thin plate made of a material having an electrooptic effect and including a portion having a thickness of 20 μm or less, and a reinforcement plate attached to a bottom surface of the thin plate and having a thickness greater than that of the thin plate.
2. Related Art Statement
Conventionally, in an optical communication field or an optical measurement field, a waveguide type optical modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electrooptic effect is much used.
Particularly, as multimedia has been developed, the amount of communication tends to increase and thus wider bandwidth of an optical modulation frequency needs to be realized. As a realizing means thereof, an external modulation method using a LiNbO3 (hereinafter, referred to as LN) modulator or the like has been diversified. However, in order to realize the wider bandwidth of the LN modulator, velocity matching between a light wave and a microwave which is a modulation signal and reduction of a driving voltage need to be realized.
As a means for solving the above-described problems, conventionally, the velocity matching has been realized by forming a buffer layer on a waveguide. However, recently, as a substrate processing technology is improved, the thickness of the substrate tries to decrease such that the velocity matching condition between the microwave and the light wave is satisfied and at the same time the driving voltage is reduced.
In Patent Documents 1 to 3, an optical waveguide and a modulation electrode are mounted on a thin substrate (hereinafter, referred to as “first substrate”) having a thickness of 30 μm or less and another substrate (hereinafter, referred to as “second substrate”) having dielectric constant lower than that of the first substrate is bonded to the first substrate, such that an effective refractive index of the microwave is lowered, the velocity matching between the microwave and the light wave is realized and mechanical strength of the substrate is maintained.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S64-18121
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-215519
[Patent Document 3] Japanese Unexamined Patent Application Publication No. H10-133159
Meanwhile, as the waveguide type optical modulator, an optical intensity modulator, an optical switch, a variable optical attenuator (VOA) or a phase modulator is developed and put into practical use. For example, in the VOA, the refractive index of the waveguide is changed by an electrooptic effect or a thermo-optic effect to adjust output light intensity, in a branch waveguide of one arm for configuring a Mach-Zehnder interferometer.
However, in a case of using a substrate having optical anisotropy, an applied voltage or the amount of heating varies depending on a polarized wave of incident light. Accordingly, only a specific polarized wave must propagate.
As a method of selecting a specific polarized wave, as shown in FIG. 1A, there is provided a method of attaching a polarizer 2 on the end surface of an entrance of a substrate 1 of an optical modulator. In a case of using an LN substrate, there is provided a method of attaching a metal film 4 on an optical waveguide 3 in an X-cut plate (see FIG. 1B) or a method of providing a slab to a side surface of an optical waveguide in a Z-cut plate.
However, when a thin plate having a thickness of 20 μm or less is used, it is difficult to attach the polarizer to the end surface of the substrate with high precision as shown in FIG. 1A. In addition, when the metal film 4 is attached to the top surface of the thin plate as shown in FIG. 1B, the substrate is at increased risk of being damaged by a difference in coefficient of thermal expansion between the substrate and metal film, and, when the metal film is formed on the optical waveguide 3, the specific polarized wave is absorbed and a light wave having a different polarization plane is also attenuated.
Since the above-described methods of selecting the specific polarized wave require a separate process in addition to a process of manufacturing the optical modulator, a manufacturing process becomes complicated and thus the manufacturing cost increases.